This invention relates to a new and improved method for manufacturing a reinforced strip having a cross section of considerably greater width than thickness with the opposing longer sides being substantially parallel and the opposing shorter sides being convex or round. More specifically it relates to a method for manufacturing a reinforced insulative strip to be used as a member to support a resistive wire. The combination of the resistive wire and the reinforced insulative strip may be used to form a ballast resistor for a gas discharge lamp such as a fluorescent lamp unit as disclosed in U.S. Pat. No. 3,996,493 Davenport et al and assigned to the same assignee as the present invention.
A current method of manufacturing ballast resistors for use in gas discharge lamp units involves helically winding a resistive wire around a strip of insulative material, placing a plurality of substantially lengthwise parallel reinforcing fibers or strands over the resistive wires on each side of the strip and then gluing the fibers onto the strip in order to reinforce the strip.
In the final assembly of a lamp unit housing of the type disclosed in the above-noted patent, the housing is extruded with the ballast resistor embedded therein such that the resistive wires are beneath the surface of the housing and approximately equidistant from the opposing surface of the holder. In one prior method, the strip of material is formed of a thermoplastic insulative material and the reinforcing strands are formed of fiberglass. The temperature of this secondary extrusion process is sufficient to soften or melt the thermoplastic insulative material of the reinforced strip of the ballast resistor. The forces involved with pulling the ballast resistor through the melt flow and the extruder during the formation of the housing are sufficient to distort and elongate an unreinforced ballast resistor. This distortion and elongation causes the given value of resistance per unit length of resistor to vary. It is because of this secondary extrusion process and the need to maintain a uniform resistance over the length of the resistor for the lamp to operate properly that the reinforcing strands are necessary.
Numerous problems have been encountered due to the positioning of the reinforcing strands on top of the resistive wires and the resultant increased thickness of the ballast resistor. Among these problems are the following: the extrusion die tends to abrade the reinforcing strands in the secondary extrusion process; the reinforcing strands are subject to being severed in the secondary extrusion process; the ballast resistor becomes positioned within the resistor holder formed by the secondary extrusion process such that the resistor is located undesirably close to an outer surface of the holder; the process of gluing reinforcing strands over the resistive wires is time consuming, expensive and does not lend itself to rapid production of the resistors as is required to increase the product output of the lamp in an assembly line environment.
Several problems are encountered when attempting to place the reinforcing strands within the thermoplastic melt flow during the extrusion of the strip. For example, the reinforcing strands, inasmuch as they are not rigid, are caused to deviate from their initial positions at the center of the extrusion die due to the pressure of the extrusion process. Such deviations cause the strands to break through the surface of the strip as the strip leaves the extruder, creating an unacceptable product and making it difficult to re-centralize the strands in the center of the extruding die in order to continue the operation.